Wind turbines are devices that convert energy from the wind to electrical energy. A typical wind turbine includes a nacelle mounted on a tower housing, a drive train for transmitting the rotation of a rotor to an electric generator and other components such as a yaw drive which orientates the wind turbine, several actuators and sensors and a brake. The rotor supports a number of blades extending radially therefrom for capturing the kinetic energy of the wind and causing the driving train rotational motion. The rotor blades have an aerodynamic shape such that when a wind blows across the surface of the blade, a lift force is generated causing the rotation of a shaft which is connected—directly or through a gearing arrangement—to the electrical generator located inside the nacelle. The amount of energy produced by wind turbines is dependent on the rotor blade sweeping surface that receives the action from the wind and consequently increasing the length of the blades leads normally to an increase of the power output of the wind turbine.
Under known control methods, the power produced by a wind turbine increases with wind speed until a rated nominal power output is reached and then it is maintained constant. In pitch regulated wind turbines, this is done regulating the pitching action of the blades in order to optimize power capture independently of the wind speed.
The wind speed is therefore an important control parameter for the operation of variable speed wind turbines. In many of the already installed wind turbines, the wind speed used by the wind turbine control system is the wind speed calculated applying a Transfer Function (hereinafter TF) to the wind speed measured by an anemometer located on the wind turbine nacelle.
The TF is defined as the function that expresses free stream wind speed as a function of wind speed measured by the wind sensor used by the wind turbine control system. This function is expected to correct the effect of the wind turbine rotor and flow distortion around the wind turbine. The TF depends on the wind turbine characteristics, on the wind sensor location and model, and on the wind characteristics in the wind turbine location.
As the wind flow around the nacelle is disturbed by the wind turbine rotor and the nacelle, it is very important to apply the appropriate TF to correct the flow distortion.
In order to obtain the appropriate TF, the free wind speed in front of the wind turbine rotor must be measured (using a cup or sonic anemometer installed on a meteorological mast, or a remote sensor of wind speed or by other means, including site calibration if necessary) and the wind speed measured by the wind turbine anemometer. The correlation function obtained, after an appropriate data analysis and corrupted data rejection, shall be applied to the data given by the wind turbine anemometer, in order to obtain the real wind speed in front of the wind turbine rotor.
A known method for obtaining an appropriate TF for a wind turbine without measuring the free wind speed in front of the wind turbine is the anemometer calibration method disclosed is US 2008/0307853 that comprises the steps of obtaining pairs of measured values of wind speed and a wind-speed dependent wind turbine variable, comparing said measured value pairs to pairs of wind speed and the turbine variable obtained from an expected turbine variable curve of the wind turbine to determine a difference between a measured wind speed value and an expected wind speed value for a given wind turbine variable value, and adjusting a calibration function of said anemometer on the basis of said determined difference.
Even though it is known the relevance of using an appropriate TF for optimizing the wind turbine power production, among the known monitoring methods for evaluating different aspects of the wind turbine performance, there is not known any specific method for monitoring the TF applied to a wind turbine.
This invention is addressed to the solution of said problem using known control means so that it can be implemented in the already installed wind turbines.